A reversal process of producing a positive dye image using as a dye image providing compound a redox dyereleaser, hereinafter also referred to simply as an RDR, and a negative-working silver halide emulsion is disclosed in Faul et al U.S. Pat. No. 3,998,637. After imagewise exposure the photographic element is developed without cross-oxidizing the RDR. The residual silver halide is then fogged, and a second development step is performed in which oxidized developing agent produced as a reaction product cross-oxidizes the RDR to permit a positive dye image to be formed. Gompf et al U.S. Pat. No. 3,938,995 and Faul et al U.K. Pat. No. 1,494,010 disclose essentially similar processes, except that the dye image providing compounds employed are, respectively, a leuco dye or a ballasted primary para-phenylenediamine. Silver halide developing agents used to cross-oxidize RDR's and other dye image providing compounds are also commonly referred to in the art as electron transfer agents, hereinafter also referred to as ETA's.
A direct reversal process for producing a positive dye image is disclosed in Hendess U.S. Pat. No. 3,647,452, wherein an imagewise exposed photographic element containing a negative-working silver halide emulsion and a dye-forming coupler is developed with a color developing agent in the presence of a competing coupler with which the oxidized color developing agent couples to form a diffusible or colorless reaction product. During continued development of residual, unexposed silver halide the competing coupler is exhausted or washed out of the material, so that the oxidized color developing agent can now couple with the dye-forming coupler to form a positive dye image.
Another color reversal process is described in Hinshaw et al U.K. Pat. No. 1,464,104, wherein a dye image providing compound is employed which releases a dye by an intramolecular nucleophilic displacement reaction at a relatively slow rate. The compound is capable of reacting more rapidly with an oxidized developing agent to prevent dye release by intramolecular nucleophilic displacement from occurring. In a preferred form intramolecular nucleophilic displacement is further delayed by employing a compound containing a hydrolyzable precursor of the nucleophilic group, thereby permitting further reduced minimum dye densities to be obtained. Direct color reversal processing can be undertaken, or, to achieve very low minimum dye densities, exposed silver halide can be developed in a first developer having a pH below that required to permit hydrolysis of the nucleophilic precursor, so that no dye is formed, while residual silver halide is developed in a second, higher pH developer so that dye is imagewise released.
A direct color reversal process is disclosed by Barr U.S. Pat. No. 3,243,294, wherein the photographic element contains a negative-working silver halide emulsion and physical development nuclei. Also incorporated in the element for black-and-white development is, in one form, a combination of a ballasted hydroquinone and a diffusible 3-pyrazolidone (also termed 3-pyrazolidinone). Upon imagewise exposure and development in the presence of a color developing agent exposed silver halide is preferentially developed by the 3-pyrazolidone and ballasted hydroquinone, so that no oxidized color developing agent and consequently no dye is produced in imagewise exposed areas. Subsequent physical development of the residual, unexposed silver halide does, however, produce oxidized color developing agent, so that a positive dye image is formed.